Analyzing Macebeth quotes: "If chance will have me king, why, chance may not crown me, / Without my stir."

Macbeth is reflecting on the Weird Sisters' prophecy and its astonishing accuracy. The witches were totally correct in predicting that Macbeth would become Thane of Glamis and Thane of Cawdor. Yet he didn't have to engage in ruthless skulduggery to obtain those honors; they were simply given to him for services rendered to the king. Macbeth starts to think that he is fated to be king, just as he was fated to be Thane of Glamis and Cawdor. The implication is that the throne will come to him easily, in much the same way as his other honors; he won't actually have to strive to be king.
What makes Macbeth such a tragic figure is that at no point does he revel in his diabolical acts. (In this regard, he couldn't be more different from Richard III, for example.) He never feels completely comfortable with killing Duncan, a man to whom he's always shown such impeccable loyalty. At this point in the play, Macbeth is torn between two conflicting impulses. On the one hand, he thinks that perhaps he should just let fate take its course—if he becomes king, good; if not, then it just wasn't meant to be. On the other hand, Macbeth can't stop thinking about murdering Duncan, about giving fate a helping hand:

If good, why do I yield to that suggestion Whose horrid image doth unfix my hair And make my seated heart knock at my ribs, Against the use of nature? (act I, scene iii).

This quote comes from act 1, scene 3 in which Macbeth and Banquo have met with the witches and heard their prophecies. While Banquo is reluctant to act the prophecies as fact, Macbeth's reaction is the opposite. As we see in this quote, the idea that Macbeth will be king is very attractive. He is already starting to think about how this might occur. The quote, therefore, links to the theme of ambition—specifically, Macbeth's ambition.
In addition, notice how Macbeth uses personification here. He depicts "chance" (or Fate) as a person, capable of making him the king of Scotland. The idea that "chance" would choose Macbeth to be king is also important because it links to the theme of fate versus free will. It suggests that Macbeth's rise to kingship might be the result of factors beyond Macbeth's control. This is a question which Shakespeare repeatedly explores in the play, prompting the reader to think deeply about Macbeth's power. Does he become king because he desperately wants it or because fate wills it?

Macbeth is saying that what happens will happen; if fate, or "chance," wants him to be king, then it will happen no matter what he does.
Macbeth has seen some of the prophecies of the witches come to pass. He is now the Thane of Glamis and Thane of Cawdor. Naturally, he wonders whether he will eventually become the king.
Macbeth isn't particularly enthused about the idea of being king. Banquo, earlier in the scene, agrees with him that it might happen as the witches foretold. Macbeth gets distracted and says to himself, "If chance will have me king, why, chance may crown me, / Without my stir." He is thinking about his chance to become king—and realizing that he may not have an option. He doesn't have to act to become king. 
If something is fated to happen, it will—whether Macbeth chooses it and works toward it or not.